An almost classic pen which adopts the above described dose setting and injecting scheme is described in EP 327 910.
By setting a dose on this pen a tubular member forming an injection button is screwed up along a threaded piston rod a distance corresponding to the distance said piston rod must be moved to inject the set dose. The tubular member simply forms a nut which is during the dose setting screwed away form a stop and which is during the injection pressed back to abutment with said stop and the force exerted on the button is directly transmitted to the a piston closing one end of an ampoule in the syringe which ampoule contains the medicament to be injected. When the piston is pressed into the ampoule the medicament is pressed out through a needle mounted through a closure at the other end of the ampoule.
By time it has been wanted to store larger amount in the ampoules, typically 3 ml instead of 1.5 ml. As it has not been appropriate to make the syringe longer the ampoule is instead given a larger diameter, i.e. the area of the piston facing the medicament in the ampoule has been doubled and consequently the force which has to be exerted on the piston to provide the same pressure as previously inside the ampoule has been doubled. Further the distance the piston has to be moved to inject one unit of the medicament has been halved.
This development is not quite favourable, as especially users having reduced finger strength have their difficulties in pressing the injection button, a problem that is further increased when still thinner needles are used to reduce the pain by injection. Also with quite small movements of the button it is difficult to feel whether the button is moved at all and by injection of one unit from a 3 ml ampoule the piston and consequently the injection button has to be moved only about 0.1 mm.
Consequently a wish for a gearing between the injection button and the piston has occurred so that the button has a larger stroke than has the piston. By such a gearing the movement of the injection button is made larger and the force, which has to be exerted on the injection button, is correspondingly reduced.
In EP 608 343 a gearing is obtained by the fact that a dose setting element is screwed up along a spindle having a thread with a high pitch. When said dose setting element is pressed back in its axial direction the thread will induce a rotation of said dose setting element, which rotation is via a coupling transmitted to a driver nut with a fine pitch which driver nut will force a threaded not rotatable piston rod forward.
A similar gearing is provided in WO 99/38554 wherein the thread with the high pitch is cut in the outer surface of a dose setting member and is engaged by a mating thread on the inner side of the cylindrical housing. However, by this kind of gearing relative large surfaces are sliding over each other so that most of the transformed force is lost due to friction between the sliding surfaces. Therefore a traditional gearing using mutual engaging gear wheels and racks is preferred.
From WO 96/26754 is known an injection device wherein two integrated gear wheels engages a rack fixed in the housing and a rack inside a plunger, respectively. When the plunger is moved axially in the housing the rack inside this plunger can drive the first gear wheel to make the other integral gear wheel move along the fixed rack in the housing. Thereby the gear wheel is moved in the direction of the plunger movement but a shorter distance than is this plunger and this axial movement of the integrated gear wheels is via a housing encompassing said gear wheels transmitted to a piston rod which presses the piston of an ampoule further into this ampoule. However, the rack inside the plunger is one of a number axial racks provided inside said plunger. These racks alternates with untoothed recesses, which allow axial movement of the plunger without the first gear wheel being in engagement with a rack in this plunger. This arrangement is provided to allow the plunger to be moved in a direction out of the housing when a dose is set. When the plunger is rotated to set a dose it is moved outward a distance corresponding to one unit during the part of the rotation where the first gear wheel passes the untoothed recess, thereafter the first gear wheel engages one of the racks so the set unit can be injected, or the rotation can be continued to make the first gear wheel pass the next recess during which passing the set dose is increased by one more unit and so on until a dose with the wanted number of units is set.
A disadvantage by this construction is that the teeth of the racks and gearwheels alternating have to be brought in and out of engagement with each other with the inherit danger of clashing. As only a few racks separated by intermediary untoothed recess can be placed along the inner surface of the plunger only few increments can be made during a 360° rotation.
In U.S. Pat. No. 6,663,602 is disclosed an injection device, wherein a direct gearing, i.e. a gearing by which more transformations of rotational movement to linear movement and linear movement to rotational movement are avoided, between the injection button and the piston rod.
The injection device shown in U.S. Pat. No. 6,663,602 comprises a housing wherein a piston rod threaded with a first pitch is non rotatable but longitudinally displaceable guided, a nut engaging the thread of the piston rod which nut can be screwed along the threaded piston rod away from a defined position in the housing to set a dose and can be pressed back to said defined position carrying the piston rod with it when the set dose is injected, a dose setting member which can be screwed outward in the housing along a thread with a second pitch to lift an injection button with it up from the proximal end of the housing. This injection device is provided with a gearbox which provides a gearing between the axial movements of the injection button and the nut relative to the housing which gearing has a gearing ratio corresponding to the ratio of said second and first pitch.
Although broadly being considered state of the art, the injection device disclosed in U.S. Pat. No. 6,663,602 presents important mechanical constraints due to the construction principle of the linear gearing system which acts during dosing. According to the injection device of U.S. Pat. No. 6,663,602, the length of the nut/piston rod assembly is increased during the dose setting by rotating the nut in a thread on the piston rod. During dosing, the linear displacement of the nut and the injection button are coupled such that there is proportionality between the movement of the nut and the injection button, the proportionality given by the transmission ratio of the gear box. As the nut is rotationally confined relatively to the piston rod during dosing, the translation of the nut corresponds to the translation of the piston rod and thus ultimately to the dose of medication administered.
As the gear is an integrated part of the construction and as the gear can only be designed to certain transmission ratios, the gear implies important constraints on the obtainable ratio of movement of the injection button relative to the movement of the piston. Due to the direct proportionality between the movement of the piston rod and the administered dose of medication, the same syringe can not easily be adapted to different medications. Although adaptation is possible by changing the transmission ratio of the linear gear, this solution will in general be very complex and imply a complete redesign of the syringe.